Synthesis and Crystal Structures of Two Monomeric Methylene Bis(diaminoalanes)

Substitution of the Cl atoms of CH2(AlCl2)2 by 2,2,6,6-tetramethylpiperidino or bis(trimethylsilyl)- amino groups leads to monomeric methylene bis(diaminoalanes). These show a wide Al-C-Al bond angle as determined by X-ray crystallography and asymmetrically bonded amino groups in the piperidino derivative.

Determination of Coupling Signs 1J(119Sn, 15N ) and 2J(119Sn, 117Sn) in Trimethylstannylamines

The l5N-labelled trimethylstannylamines 1 - 3 [(Me3Sn)3N, (Me3Sn)2NPh, (Me3Sn)2NBCgHi4 (BCgHi4 = 9-borabicyclo[3.3.0]nonyn and the non-labelled 4, Me3Sn-N(BC8Hi4)2 , were prepared and studied by 1H, 1C, 15N and 119Sn NMR. The 15N ultrahigh resolution NMR spectra of 1 revealed otherwise unobserved parameters such as 2J(15N ,Sn ,13C ) and the isotope induced chemical shift 2Δ12/13C(15N). 119Sn NMR spectra of 1, recorded under similar conditions, also show new parameters such as 3J(119Sn,N,Sn,13C) which are not resolved in the non-labelled derivative. By using various types o f two-dimensional heteronuclear shift correlations, absolute coupling signs of 1J(119Sn,15N) (all < 0) in 1 - 3 were determined. By the same techniques it proved possible to confirm the negative sign o f :7(Sn,Sn) (-195.4 Hz) in 1. In contrast, the coupling constants 2J(Sn,Sn) for 2 (+71.7) and 3 (+62.0) possess a positive sign. This sign inversion, observed here for the first time for apparently similar compounds, demonstrates the enormous influence of substituents on the nature of the lone pair of electrons at the nitrogen atom. It also shows that these experiments for sign determinations of coupling constants are necessary in order to interpret these data correctly.

Über das Octastannandiid R*6Sn8[Na(THF)2 ]2 und zur möglichen Existenz des Octastannans R*6Sn8 [1] / On an Octastannanediide R*6Sn8[Na(THF)2]2 and the Possible Existence of an Octastannane R*6Sn8 [1]

The black, air and moisture sensitive octastannanediide R* 6Sn8[Na(THF)2]2 (4) (R* = SitBu3) is prepared by reaction of Sn[N(SiMe3)2]2 with R*Na in tBuOMe at -78°C. Red brown octastannane R*6Sn8 (5) is formed by thermolysis of the tristannacyclopropene R*4Sn3 in benzene at 100 °C. According to the result of the X-ray structural analysis, 4 contains a Sn8 cubane with six Sn atoms each connected with R* and two at the end of a space diagonal connected with Na(THF)2. A preliminary X-ray structure analysis shows 5 to adopt the same Sn8 cubane structure as 4 with two bare Sn atoms at the end of a space diagonal.

Glycopyrrolate does not influence the visual or motor-induced increase in regional cerebral perfusion

Acetylcholine may contribute to the increase in regional cerebral blood flow (rCBF) during cerebral activation since glycopyrrolate, a potent inhibitor of acetylcholine, abolishes the exercise-induced increase in middle cerebral artery mean flow velocity. We tested the hypothesis that cholinergic vasodilatation is important for the increase in rCBF during cerebral activation. The subjects were 11 young healthy males at an age of 24 ± 3 years (mean ± SD). We used arterial spin labeling and blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) to evaluate rCBF with and without intravenous glycopyrrolate during a handgrip motor task and visual stimulation. Glycopyrrolate increased heart rate from 56 ± 9 to 114 ± 14 beats/min (mean ± SD; p < 0.001), mean arterial pressure from 86 ± 8 to 92 ± 12 mmHg, and cardiac output from 5.6 ± 1.4 to 8.0 ± 1.7 l/min. Glycopyrrolate had, however, no effect on the arterial spin labeling or BOLD responses to the handgrip motor task or to visual stimulation. This study indicates that during a handgrip motor task and visual stimulation, the increase in rCBF is unaffected by blockade of acetylcholine receptors by glycopyrrolate. Further studies on the effect of glycopyrrolate on middle cerebral artery diameter are needed to evaluate the influence of glycopyrrolate on mean flow velocity during intense exercise.

Acute volume expansion attenuates hyperthermia-induced reductions in cerebral perfusion during simulated hemorrhage

Hyperthermia reduces the capacity to withstand a simulated hemorrhagic challenge, but volume loading preserves this capacity. This study tested the hypotheses that acute volume expansion during hyperthermia increases cerebral perfusion and attenuates reductions in cerebral perfusion during a simulated hemorrhagic challenge induced by lower-body negative pressure (LBNP). Eight healthy young male subjects underwent a supine baseline period (pre-LBNP), followed by 15- and 30-mmHg LBNP while normothermic, hyperthermic (increased pulmonary artery blood temperature ~1.1°C), and following acute volume infusion while hyperthermic. Primary dependent variables were mean middle cerebral artery blood velocity (MCAvmean), serving as an index of cerebral perfusion; mean arterial pressure (MAP); and cardiac output (thermodilution). During baseline, hyperthermia reduced MCAvmean (P = 0.001) by 12 ± 9% relative to normothermia. Volume infusion while hyperthermic increased cardiac output by 2.8 ± 1.4 l/min (P < 0.001), but did not alter MCAvmean (P = 0.99) or MAP (P = 0.39) compared with hyperthermia alone. Relative to hyperthermia, at 30-mmHg LBNP acute volume infusion attenuated reductions (P < 0.001) in cardiac output (by 2.5 ± 0.9 l/min; P < 0.001), MAP (by 5 ± 6 mmHg; P = 0.004), and MCAvmean (by 12 ± 13%; P = 0.002). These data indicate that acute volume expansion does not reverse hyperthermia-induced reductions in cerebral perfusion pre-LBNP, but that it does attenuate reductions in cerebral perfusion during simulated hemorrhage in hyperthermic humans.

Cerebral perfusion, oxygenation and metabolism during exercise in young and elderly individuals

We evaluated cerebral perfusion, oxygenation and metabolism in 11 young (22 ± 1 years) and nine older (66 ± 2 years) individuals at rest and during cycling exercise at low (25% W(max)), moderate (50% Wmax), high (75% W(max)) and exhaustive (100% W(max)) workloads. Mean middle cerebral artery blood velocity (MCA V(mean)), mean arterial pressure (MAP), cardiac output (CO) and partial pressure of arterial carbon dioxide (P(aCO2)) were measured. Blood samples were obtained from the right internal jugular vein and brachial artery to determine concentration differences for oxygen (O2), glucose and lactate across the brain. The molar ratio between cerebral uptake of O2 versus carbohydrate (O2-carbohydrate index; O2/[glucose + 1/2 lactate]; OCI), the cerebral metabolic rate of O2 (CMRO2) and changes in mitochondrial O2 tension ( P(mitoO2)) were calculated. 100% W(max) was ~33% lower in the older group. Exercise increased MAP and CO in both groups (P < 0.05 vs. rest), but at each intensity MAP was higher and CO lower in the older group (P < 0.05). MCA V(mean), P(aCO2) and cerebral vascular conductance index (MCA V(mean)/MAP) were lower in the older group at each exercise intensity (P < 0.05). In contrast, young and older individuals exhibited similar increases in CMRO2 (by ~30 μmol (100 g(-1)) min(-1)), and decreases in OCI (by ~1.5) and (by ~10 mmHg) during exercise at 75% W(max). Thus, despite the older group having reduced cerebral perfusion and maximal exercise capacity, cerebral oxygenation and uptake of lactate and glucose are similar during exercise in young and older individuals.

Pulmonary artery and intestinal temperatures during heat stress and cooling

PURPOSE

In humans, whole body heating and cooling are used to address physiological questions where core temperature is central to the investigated hypotheses. Core temperature can be measured in various locations throughout the human body. The measurement of intestinal temperature is increasingly used in laboratory settings as well as in athletics. However, it is unknown whether intestinal temperature accurately tracks pulmonary artery blood temperature, the gold standard, during thermal stimuli in resting humans, which is the investigated hypothesis.

METHODS

This study compared pulmonary artery blood temperature (via thermistor in a pulmonary artery catheter) with intestinal temperature (telemetry pill) during whole body heat stress (n = 8), followed by whole body cooling in healthy humans (mean ± SD; age = 24 ± 3 yr, height = 183 ± 8 cm, mass = 78.1 ± 8.2 kg). Heat stress and subsequent cooling were performed by perfusing warm followed by cold water through a tube-lined suit worn by each subject.

RESULTS

Before heat stress, blood temperature (36.69°C ± 0.25°C) was less than intestinal temperature (36.96°C ± 0.21°C, P = 0.004). The increase in blood temperature after 20 min of heat stress was greater than the intestinal temperature (0.70 ± 0.24 vs 0.47 ± 0.18, P = 0.001). However, the increase in temperatures at the end of heat stress was similar between sites (blood Δ = 1.32°C ± 0.20°C vs intestinal Δ = 1.21°C ± 0.36°C, P = 0.30). Subsequent cooling decreased blood temperature (Δ = -1.03°C ± 0.34°C) to a greater extent than intestinal temperature (Δ = -0.41°C ± 0.30°C, P = 0.04).

CONCLUSIONS

In response to the applied thermal provocations, early temperature changes in the intestine are less than the temperature changes in pulmonary artery blood.

Hypoxia and exercise provoke both lactate release and lactate oxidation by the human brain

Lactate is shuttled between organs, as demonstrated in the Cori cycle. Although the brain releases lactate at rest, during physical exercise there is a cerebral uptake of lactate. Here, we evaluated the cerebral lactate uptake and release in hypoxia, during exercise and when the two interventions were combined. We measured cerebral lactate turnover via a tracer dilution method ([1-(13)C]lactate), using arterial to right internal jugular venous differences in 9 healthy individuals (5 males and 4 females), at rest and during 30 min of submaximal exercise in normoxia and hypoxia (F(i)o(2) 10%, arterial oxygen saturation 72 ± 10%, mean ± sd). Whole-body lactate turnover increased 3.5-fold and 9-fold at two workloads in normoxia and 18-fold during exercise in hypoxia. Although middle cerebral artery mean flow velocity increased during exercise in hypoxia, calculated cerebral mitochondrial oxygen tension decreased by 13 mmHg (P<0.001). At the same time, cerebral lactate release increased from 0.15 ± 0.1 to 0.8 ± 0.6 mmol min(-1) (P<0.05), corresponding to ∼10% of cerebral energy consumption. Concurrently, cerebral lactate uptake was 1.0 ± 0.9 mmol min(-1) (P<0.05), of which 57 ± 9% was oxidized, demonstrating that lactate oxidation may account for up to ∼33% of the energy substrate used by the brain. These results support the existence of a cell-cell lactate shuttle that may involve neurons and astrocytes.

Effect of heat stress on cardiac output and systemic vascular conductance during simulated hemorrhage to presyncope in young men

During moderate actual or simulated hemorrhage, as cardiac output decreases, reductions in systemic vascular conductance (SVC) maintain mean arterial pressure (MAP). Heat stress, however, compromises the control of MAP during simulated hemorrhage, and it remains unknown whether this response is due to a persistently high SVC and/or a low cardiac output. This study tested the hypothesis that an inadequate decrease in SVC is the primary contributing mechanism by which heat stress compromises blood pressure control during simulated hemorrhage. Simulated hemorrhage was imposed via lower body negative pressure (LBNP) to presyncope in 11 passively heat-stressed subjects (increase core temperature: 1.2 ± 0.2°C; means ± SD). Cardiac output was measured via thermodilution, and SVC was calculated while subjects were normothermic, heat stressed, and throughout subsequent LBNP. MAP was not changed by heat stress but was reduced to 45 ± 12 mmHg at the termination of LBNP. Heat stress increased cardiac output from 7.1 ± 1.1 to 11.7 ± 2.2 l/min (P < 0.001) and increased SVC from 0.094 ± 0.018 to 0.163 ± 0.032 l·min(-1)·mmHg(-1) (P < 0.001). Although cardiac output at the onset of syncopal symptoms was 37 ± 16% lower relative to pre-LBNP, presyncope cardiac output (7.3 ± 2.0 l/min) was not different than normothermic values (P = 0.46). SVC did not change throughout LBNP (P > 0.05) and at presyncope was 0.168 ± 0.044 l·min(-1)·mmHg(-1). These data indicate that in humans a cardiac output adequate to maintain MAP while normothermic is no longer adequate during a heat-stressed-simulated hemorrhage. The absence of a decrease in SVC at a time of profound reductions in MAP suggests that inadequate control of vascular conductance is a primary mechanism compromising blood pressure control during these conditions.

Colloid volume loading does not mitigate decreases in central blood volume during simulated haemorrhage while heat stressed

Heat stress results in profound reductions in the capacity to withstand a simulated haemorrhagic challenge; however, this capacity is normalized if the individual is volume loaded prior to the challenge. The present study tested the hypothesis that volume loading during passive heat stress attenuates the reduction in regional blood volumes during a simulated haemorrhagic challenge imposed via lower-body negative pressure (LBNP). Seven subjects underwent 30 mmHg LBNP while normothermic, during passive heat stress (increased internal temperature ∼1◦C), and while continuing to be heated after intravenous colloid volume loading (11 ml kg⁻¹). Relative changes in torso and regional blood volumes were determined by gamma camera imaging with technetium-99m labelled erythrocytes. Heat stress reduced blood volume in all regions (ranging from 7 to 16%), while subsequent volume loading returned those values to normothermic levels. While normothermic,LBNP reduced blood volume in all regions (torso: 22 ± 8%; heart: 18 ± 6%; spleen: 15 ± 8%). During LBNP while heat stressed, the reductions in blood volume in each region were markedly greater when compared to LBNP while normothermic (torso: 73 ± 2%; heart: 72 ± 3%; spleen: 72 ± 5%, all P<0.001 relative to normothermia). Volume loading during heat stress did not alter the extent of the reduction in these blood volumes to LBNP relative to heat stress alone (torso: 73 ± 1%; heart: 72 ± 2%; spleen: 74 ± 3%, all P>0.05 relative to heat stress alone). These data suggest that blood volume loading during passive heat stress (via 11 ml kg⁻¹ of a colloid solution) normalizes regional blood volumes in the torso, but does not mitigate the reduction in central blood volume during a simulated haemorrhagic challenge combined with heat stress.

Sympathetic influence on cerebral blood flow and metabolism during exercise in humans

This review focuses on the possibility that autonomic activity influences cerebral blood flow (CBF) and metabolism during exercise in humans. Apart from cerebral autoregulation, the arterial carbon dioxide tension, and neuronal activation, it may be that the autonomic nervous system influences CBF as evidenced by pharmacological manipulation of adrenergic and cholinergic receptors. Cholinergic blockade by glycopyrrolate blocks the exercise-induced increase in the transcranial Doppler determined mean flow velocity (MCA Vmean). Conversely, alpha-adrenergic activation increases that expression of cerebral perfusion and reduces the near-infrared determined cerebral oxygenation at rest, but not during exercise associated with an increased cerebral metabolic rate for oxygen (CMRO(2)), suggesting competition between CMRO(2) and sympathetic control of CBF. CMRO(2) does not change during even intense handgrip, but increases during cycling exercise. The increase in CMRO(2) is unaffected by beta-adrenergic blockade even though CBF is reduced suggesting that cerebral oxygenation becomes critical and a limited cerebral mitochondrial oxygen tension may induce fatigue. Also, sympathetic activity may drive cerebral non-oxidative carbohydrate uptake during exercise. Adrenaline appears to accelerate cerebral glycolysis through a beta2-adrenergic receptor mechanism since noradrenaline is without such an effect. In addition, the exercise-induced cerebral non-oxidative carbohydrate uptake is blocked by combined beta 1/2-adrenergic blockade, but not by beta1-adrenergic blockade. Furthermore, endurance training appears to lower the cerebral non-oxidative carbohydrate uptake and preserve cerebral oxygenation during submaximal exercise. This is possibly related to an attenuated catecholamine response. Finally, exercise promotes brain health as evidenced by increased release of brain-derived neurotrophic factor (BDNF) from the brain.

Plasma pH does not influence the cerebral metabolic ratio during maximal whole body exercise

Exercise lowers the cerebral metabolic ratio of O2 to carbohydrate (glucose+1/2 lactate) and metabolic acidosis appears to promote cerebral lactate uptake. However, the influence of pH on cerebral lactate uptake and, in turn, on the cerebral metabolic ratio during exercise is not known. Sodium bicarbonate (Bicarb, 1 M; 350-500 ml) or an equal volume of normal saline (Sal) was infused intravenously at a constant rate during a '2000 m' maximal ergometer row in six male oarsmen (23±2 years; mean±S.D.). During the Sal trial, pH decreased from 7.41±0.01 at rest to 7.02±0.02 but only to 7.36±0.02 (P <0.05) during the Bicarb trial. Arterial lactate increased to 21.4±0.8 and 32.7±2.3 mM during the Sal and Bicarb trials, respectively (P <0.05). Also, the arterial-jugular venous lactate difference increased from-0.03±0.01 mM at rest to 3.2±0.9 mM (P <0.05) and 3.4±1.4 mM (P <0.05) following the Sal and Bicarb trials, respectively. Accordingly, the cerebral metabolic ratio decreased equally during the Sal and Bicarb trials: from 5.8±0.6 at rest to 1.7±0.1 and 1.8±0.2, respectively. The enlarged blood-buffering capacity after infusion of Bicarb eliminated metabolic acidosis during maximal exercise but that did not affect the cerebral lactate uptake and, therefore, the decrease in the cerebral metabolic ratio.

Modelflow underestimates cardiac output in heat-stressed individuals

An estimation of cardiac output can be obtained from arterial pressure waveforms using the Modelflow method. However, whether the assumptions associated with Modelflow calculations are accurate during whole body heating is unknown. This project tested the hypothesis that cardiac output obtained via Modelflow accurately tracks thermodilution-derived cardiac outputs during whole body heat stress. Acute changes of cardiac output were accomplished via lower-body negative pressure (LBNP) during normothermic and heat-stressed conditions. In nine healthy normotensive subjects, arterial pressure was measured via brachial artery cannulation and the volume-clamp method of the Finometer. Cardiac output was estimated from both pressure waveforms using the Modeflow method. In normothermic conditions, cardiac outputs estimated via Modelflow (arterial cannulation: 6.1 ± 1.0 l/min; Finometer 6.3 ± 1.3 l/min) were similar with cardiac outputs measured by thermodilution (6.4 ± 0.8 l/min). The subsequent reduction in cardiac output during LBNP was also similar among these methods. Whole body heat stress elevated internal temperature from 36.6 ± 0.3 to 37.8 ± 0.4°C and increased cardiac output from 6.4 ± 0.8 to 10.9 ± 2.0 l/min when evaluated with thermodilution (P < 0.001). However, the increase in cardiac output estimated from the Modelflow method for both arterial cannulation (2.3 ± 1.1 l/min) and Finometer (1.5 ± 1.2 l/min) was attenuated compared with thermodilution (4.5 ± 1.4 l/min, both P < 0.01). Finally, the reduction in cardiac output during LBNP while heat stressed was significantly attenuated for both Modelflow methods (cannulation: -1.8 ± 1.2 l/min, Finometer: -1.5 ± 0.9 l/min) compared with thermodilution (-3.8 ± 1.19 l/min). These results demonstrate that the Modelflow method, regardless of Finometer or direct arterial waveforms, underestimates cardiac output during heat stress and during subsequent reductions in cardiac output via LBNP.

Effect of volume loading on the Frank-Starling relation during reductions in central blood volume in heat-stressed humans

During reductions in central blood volume while heat stressed, a greater decrease in stroke volume (SV) for a similar decrease in ventricular filling pressure, compared to normothermia, suggests that the heart is operating on a steeper portion of a Frank-Starling curve. If so, volume loading of heat-stressed individuals would shift the operating point to a flatter portion of the heat stress Frank-Starling curve thereby attenuating the reduction in SV during subsequent decreases in central blood volume. To investigate this hypothesis, right heart catheterization was performed in eight males from whom pulmonary capillary wedge pressure (PCWP), central venous pressure and SV (via thermodilution) were obtained while central blood volume was reduced via lower-body negative pressure (LBNP) during normothermia, whole-body heating (increase in blood temperature 1 degrees C), and during whole-body heating after intravascular volume expansion. Volume expansion was accomplished by administration of a combination of a synthetic colloid (HES 130/0.4, Voluven) and saline. Before LBNP, SV was not affected by heating (122 +/- 30 ml; mean +/- s.d.) compared to normothermia (110 +/- 20 ml; P = 0.06). However, subsequent volume loading increased SV to 143 +/- 29 ml (P = 0.003). LBNP provoked a larger decrease in SV relative to the decrease in PCWP during heating (8.6 +/- 1.9 ml mmHg(1)) compared to normothermia (4.5 +/- 3.0 ml mmHg(1), P = 0.02). After volume loading while heat stressed, the reduction in the SV to PCWP ratio during LBNP was comparable to that observed during normothermia (4.8 +/- 2.3 ml mmHg(1); P = 0.78). These data support the hypothesis that a Frank-Starling mechanism contributes to compromised blood pressure control during simulated haemorrhage in heat-stressed individuals, and extend those findings by showing that volume infusion corrects this deficit by shifting the operating point to a flatter portion of the heat stress Frank-Starling curve.